A curable silicone composition, which can be cured by a hydrosilylation reaction, is used for various applications because it forms a cured product having excellent rubber characteristics such as hardness and elongation. Known examples of curable silicone compositions include: a curable silicone composition including: a straight-chain organopolysiloxane having at least two alkenyl groups in a molecule; an organopolysiloxane comprising SiO4/2 units, ViR2SiO1/2 units, and R3SiO1/2 units, where Vi is a vinyl group, and R is a substituted or unsubstituted monovalent hydrocarbon group having no unsaturated aliphatic bond; an organopolysiloxane having at least one silicon-bonded alkoxy group and at least two silicon-bonded hydrogen atoms in a molecule; and a platinum metal-based catalyst (see Patent Document 1) and a curable silicone composition including: a straight-chain organopolysiloxane having at least two alkenyl groups in a molecule; an organopolysiloxane comprising SiO4/2 units and R′(CH3)2SiO1/2 units, where R′ is either an alkenyl group or a methyl group, and having at least three alkenyl groups in a molecule; an organopolysiloxane comprising SiO4/2 units and R″(CH3)2SiO1/2 units, where R″ is either a hydrogen atom or a methyl group, and having at least three silicon-bonded hydrogen atoms in a molecule; and a platinum group metal compound (see Patent Documents 2 and 3).
These curable silicone compositions are suitable as a sealing agent for an optical semiconductor device, producing no byproducts and forming transparent cured materials. However, there is a problem of crack formation in the cured product, leading to degradation in adhesion and sealing performance with a substrate, and a decrease in transparency, when used in the environment with the temperature not less than 200° C. for a long duration.
Meanwhile, it is known that the addition of an organopolysiloxane containing cerium to a curable silicone composition improves the heat resistance thereof (e.g. see Patent Documents 4 and 5). However, the organopolysiloxane containing cerium exhibits yellow color and has never been used as a curable silicone composition for an optical semiconductor device, where transparency is required.